In this laboratory, we have been working on hormonal factors which might affect the rate of active transport in the thick ascending limb of Henle's loop. Vasopressin has been found to increase the electrical potential difference and the rate of active chloride transport in the mouse medullary thick ascending limb, but in contrast to its action in the collecting tubule, it has no effect on water permeability in this segment. A vasopressin sensitive adenyl cyclase has been found in this segment and exogenous cyclic AMP can reproduce the effect. In more recent work, beta-adrenergic agents have also been found to stimulate the rate of active chloride transport in this segment as indicated by an increased luminal positive voltage, and increased rate of chloride efflux and an unchanged rate of chloride influx. No additive effect of beta-adrenergic agents and exogenous cyclic AMP was observed, a finding consistent with a cyclic AMP mediated response. However, no beta-adrenergic sensitive adenyl cyclase has been observed in this segment. Preliminary results with trifluoperazine have suggested a possible role for calcium or calmodulin in the response to both beta-adrenergic agents and vasopressin. Present work is directed at establishing this possible link in the vasopressin and beta-adrenergic stimulation of transport in this part of the nephron.